1. Field of the Invention
The present invention relates to a bump bonding method and apparatus used when fabricating a flip-chip semiconductor integrated circuit (hereinafter, referred to as an IC), for forming a protruding electrode (hereinafter, referred to as a stud bump or simply as a bump) on the IC.
2. Description of Related Art
Conventionally, a bump bonding technique is known which applies the wire bonding technique to form a bump. The bump is formed by the ultrasonic joining of a ball, made of gold or gold-plated metal, to an electrode pad (hereinafter, referred to simply as a pad) on the flip-chip IC.
FIG. 7A and FIG. 7B show a typical embodiment of a conventional bump bonding apparatus. Referring to FIG. 7A and FIG. 7B, reference numeral 31 denotes a gold wire, reference numeral 32 denotes a capillary through which the gold wire 31 penetrates, reference numeral 33 denotes an IC, reference numeral 34 denotes a discharge torch provided in close proximity to the tip of the gold wire 31, reference numeral 35 denotes a ultrasonic horn having the capillary 32 attached at the tip portion thereof, and reference numeral 36 denotes a ultrasonic transducer attached to the base portion of the ultrasonic horn 35. Reference numeral 37 denotes a bonding arm whose middle portion is pivotably supported by a supporting bracket 38 through a pivotal supporting axis 37a. The bonding arm 37 is provided with the ultrasonic horn 35 and the ultrasonic transducer 36 at one end, and with vertical driving means 39 at the other end. Reference numeral 40 denotes an X-Y table on which is placed the supporting bracket 38. Reference numeral 41 denotes a damper for holding the gold wire 31, and reference numeral 42 denotes an air tensioner for lifting the gold wire 31. Reference numeral 43 denotes a heat stage for heating the IC 33. The heat stage 43 also has a function for positioning and holding the IC 33. Reference numeral 44 denotes a heater of the heat stage 43. Reference numeral 45 denotes a recognition camera for recognizing the position of the IC 33. Reference numeral 46 denotes an ultrasonic oscillator, reference numeral 47 denotes a spark generator, and reference numeral 48 denotes a high voltage cable.
The operation of the conventional bump bonding apparatus will now be explained. Initially, the IC 33 is held and heated at the heat stage 43. Meanwhile, the recognition camera 45 recognizes the IC 33, and then the relative position of the IC 33 with respect to the capillary 32 is determined through the use of the X-Y table 40. The spark generator 47 is activated to generate a spark between the tip of the gold wire 31 (protruding from the capillary 32) and the discharge torch 34, to form a gold ball 49 as shown in FIG. 8A. The vertical driving means 39 then drives the bonding arm 37 so as to lower the capillary 32. After an abutment detecting function furnished to the vertical driving means 39 detects a pad portion 50 on the IC 33, the ultrasonic oscillator 46 starts to drive the ultrasonic transducer 36 while a specific applied pressure is maintained on the gold ball 49. This oscillates ultrasonic waves at a frequency in the range of 64 to 110 kHz, through the ultrasonic horn 35, to the capillary 32. Then, as shown in FIG. 8B, the gold ball 49 is joined to the pad portion 50 on the IC 33, thereby forming a bump 51. Subsequently, after the capillary 32 is elevated by a certain distance, the gold wire 31 is elevated while being held by the damper 41, thereby causing the joint portion between the bump 51 and the gold wire 31 to break. The bump 51, formed as a protruding electrode is thus completed over the pad portion 50 on the IC 33.
When a bump is formed as described above, the heating temperature of the heat stage 43 by the heater 44 is determined depending on the size of the bump 51 to be formed, and the kind of the IC 33 to be used. To be more specific, the temperature is set to 260° C. when the diameter of a bump pedestal 51a (see FIG. 8B) is 80 μm, and to around 300° C. when it is 50 μm. This is because the microscopic joint area is reduced when the bump pedestal 51a becomes smaller. As the energy generated from the ultrasonic waves and load cannot be applied to such a small bump pedestal at such a high level, it makes it necessary to apply more energy from the outside.
Also, in the case of an IC incorporated in a SAW filter or a sensor having poor resistance to heat, the temperature cannot be raised as high as specified above. Consequently, heat is supplied only up to a level less than the upper temperature which the IC is resistant.
In recent years, however, the distance between adjacent electrode pad portions on the IC has been reduced, and so is the size of the bump pedestal 51a. This makes it necessary to raise the heating temperature of the IC 33. Raising the heating temperature, however, poses three major problems as follows:
A first problem is that because the heating temperature cannot be raised for an IC having poor resistance to heat, it is difficult to form a bump having high reliability on such an IC.
A second problem is that heat from the heater 44 adversely effects the bonding mechanism portion. Heat propagated from the heater 44 to the heat stage 43 is released and is eventually transferred to the bonding mechanism portion, which gives rise to thermal expansion of the horn 35, the lens-barrel portion of the recognition camera 45, and the bonding arm 37. As a consequence, bonding is provided at a position offset from the position judged by the recognition camera 45, resulting in the bump 51 extending out from the pad portion 50. When the distance present between the adjacent pad potions 50 is small, the bump 51 can touch an adjacent bump 51, causing a connection defect to occur when joined to the substrate in a latter step of the sequence.
A third problem is the influence upon the bonding process on a wafer. In order to make the IC thinner and reduce the step numbers involved in the dice and pick process, the process of forming bumps on a wafer before it is cut into ICs has become mainstream in recent years in contrast to the process of forming bumps on individual ICs. According to this method, however, when the wafer is heated by the heater 44, applied heat is maintained in the ICs after the bumps are formed thereon. As a result, more metal is diffused between the gold balls and the pad portions made of aluminum. This causes cracking or voids on the joint interface, which again results in a joint defect. Also, in the case of a wafer attached to a sheet, the heating temperature of the heater 44 is limited to, or below, the temperature to which the sheet is resistant. This makes it difficult to maintain the joint quality.